The present invention relates to a radial tire, more specifically, a radial tire which can save fuel consumption of a car.
Recently, global scale environmental problems such as ozone layer depletion, acid rain and greenhouse effect of the earth, come into question. These problems mostly derive from components in exhaust gases produced in combustion of car fuels such as gasoline. Therefore, reducing the combustion amount of car fuels can be one effective resolution against these problems.
Accordingly, in North America, laws concerning fuel consumption such as Gas Guzzler Tax and Company Average Fuel Economy (CAFE), for example, are under consideration. As technical developments according to CAFE, there are studied improvement of efficiency of an engine or the like, lowering of the energy loss of each part of a car, lowering of the weight of a car, and the like. With respect to a tire, the above-mentioned problems can be resolved by accomplishing, at the same time, lowering of energy loss in a tire and lowering of weight of a tire.
In order to lower the energy loss of a tire, several approaches are employed. For Example, a tan .delta. peak temperature of a cap tread is lowered (Approach 1-1). When the tan .delta. peak temperature of a cap tread is lowered, however, wet-grip of the cap tread tends to be lowered. Also, for the same purpose, a carbon black content of the cap tread is lowered (Approach 1-2). When the carbon black content is lowered, however, the wet grip and the dry grip tend to be lowered and the abrasion resistance also tends to be lowered. Further, a cap/base-structure is adopted for a tread and a carbon black content of a base tread rubber is lowered (Approach 2). When the carbon black content of a base tread rubber is lowered, however, the rigidity of the base tread tends to be lowered to make the steering stability worse. Still further, in order to lower the energy loss of a side wall rubber, a carbon black content of the side wall rubber is lowered (Approach 3). When the carbon black content of the side wall rubber is lowered, however, the rigidity of the side wall tends to be lowered to make the steering stability of a car worse.
In order to lower a weight of a tire, a groove depth of a cap tread is made shallow (Approach 4). When the groove depth is made shallow, however, the tire life tends to be shorten. Also, for the same purpose, the thickness of a side wall is made small (Approach 5). When the thickness of a side wall is made small, however, the ridigity of a tire tends to be deficient to make the steering stability worse. Further, to lower the weight of a tire, the volume of a bead apex is made small (Approach 6). When the volume of the bead apex is made small, however, the steering stability of a car tends to be worse.
Further, by combining the Approaches 1-1, 2-2 to 6, a tire which has a low energy loss and a low weight is obtained to realize the lowering of fuel consumption of a car. However, the obtained tire is inferior in steering stability such as grip characteristic and handle responsibility.
Also, there is proposed a tire which contains a short fiber in each component such as a base tread, a side wall or a bead filler for improving the steering stability of a car (JP-A-204637/1984, JP-A-204638/1984, JP-A-25605/1986, JP-A-263104/1988). However, the tire cannot realize the lowering of energy loss and the lowering of weight.
The present invention has been accomplished to solve the above problems, and provides a radial tire which has a low energy loss, has a low weight and realize excellent steering stability for a car.